WIP for preprints remodel: Supplementary files #1590 take 2

[tingletech]

supersedes #1749

second attempt at #1590

this sort of works, but is not quite finished

I didn't commit the migration, so run python src/manage.py makemigrations

[Bbkctp]

Can one of the admins verify this patch?

[tingletech]

the sequence is not hooked up right

mysql> select url, label, sequence from repository_preprintsupplementaryfile where preprint_id=1429;
+--------------------+--------+----------+
| url                | label  | sequence |
+--------------------+--------+----------+
| http://1.link.com/ | Link 1 |        1 |
| http://2.link.com/ | Link 2 |        1 |
+--------------------+--------+----------+
2 rows in set (0.00 sec)

also needs better error handling

[ajrbyers]

test this please